I've been using Mouse Without Borders for a couple of months now. Mouse Without Borders lets you control multiple PCs with one keyboard and mouse. It's easy to install and configure. It's still classified as prototype, but I haven't had any issues so far. I have it running on three flavors of Windows (7, Vista, XP). I've used Multiplicity and Synergy in the past to span PCs, but this time around I'm really liking Mouse Without Borders.
Microsoft Garage Mouse without Borders
Tip: be sure to check Wrap Mouse (Other Options tab). If you shutdown a PC in the middle of your chain, you won't be able to get to the next PC after it. Turning wrap on will let you go the other direction to get those PCs.
If you want to span Windows, Mac, and Linux workstations take a look at Synergy. Multiplicity and Mouse without Borders are Windows only.